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April 29, 2009

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“Out of Africa” came mostly men, geneticists say

Dec. 21, 2008
Courtesy Har­vard Med­i­cal School
and World Science staff

Many sci­en­tists be­lieve an­a­tom­ic­ally mod­ern hu­mans left Af­ri­ca over 60,000 years ago, in a migra­t­ion re­spon­si­ble for pro­duc­ing nearly all hu­man popula­t­ions out­side Af­ri­ca to­day.

Now, re­search­ers say men out­num­bered wom­en in that ex­o­dus, tra­di­tion­ally dubbed the “out of Af­ri­ca” sce­na­rio.

In a stu­dy, sci­en­tists an­a­lyzed da­ta on X chro­mo­some, one of two sex-de­ter­min­ing chro­mo­somes in hu­mans. Males have one X chro­mo­some, fe­males two. The in­ves­ti­gat­ors traced geo­graph­i­cal varia­t­ions in the X chro­mo­some and in the non-sex chro­mo­somes.  The stu­dy, by re­search­ers at Har­vard Med­i­cal School and the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy, is pub­lished in the Dec. 21 is­sue of the re­search jour­nal Na­ture Ge­net­ics.

The re­search­ers said they weren’t sure why more men than wom­en would have par­ti­ci­pated in the dis­per­sion from Af­ri­ca. 

But the lead au­thor, Har­vard’s Alon Keinan, noted the find­ings are “in line with what an­thro­po­l­o­gists have taught us about hunter-gatherer popula­t­ions, in which short dis­tance migra­t­ion is pri­marily by wom­en and long dis­tance migra­t­ion pri­marily by men.”

The study drew on the fact that in a giv­en popula­t­ion, the frac­tion of all chro­mo­somes that are X chro­mo­somes changes de­pend­ing on the pro­por­tion of men. That in turn af­fects the rate at which muta­t­ions ran­domly spread through the X chro­mo­somes rel­a­tive to oth­er chro­mo­somes. One can de­tect the re­sults of these pro­cesses by com­par­ing the fre­quen­cies of spe­cif­ic gene vari­ants in mod­ern popula­t­ions.

The sci­en­tists say their meth­od of com­par­ing X chro­mo­somes with the oth­er non-gender spe­cif­ic chro­mo­somes will be a pow­er­ful tool for fu­ture his­tor­i­cal and an­thro­po­log­ical stud­ies, since it can il­lu­mi­nate dif­fer­ences in fe­male and male popula­t­ions that were in­ac­ces­si­ble to pre­vi­ous meth­ods. 

Keinan and col­leagues not­ed that a few oth­er pro­cesses could ex­plain their find­ings, in­clud­ing the idea that a ti­ny frac­tion of the wom­en bore nearly all the chil­dren dur­ing the out-of-Af­ri­ca dis­per­sal. These seem un­like­ly, they added, but “fu­ture work should ex­plore these sce­nar­ios,” they wrote.

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Scientists believe anatomically modern humans left Africa over 60,000 years ago in a migration thought to be responsible for producing nearly all human populations outside Africa today. Now, researchers say men and women probably weren’t equal partners in that exodus. In a study, scientists analyzed data on X chromosome, one of two sex-determining chromosomes in humans. Males have an X chromosome, females two. By tracing geographical variations in the X chromosome and in the non-sex chromosomes, the researchers found evidence that men likely outnumbered women in that migration. The study, by researchers at Harvard Medical School and the Massachusetts Institute of Technology, is published in the Dec. 21 issue of the research journal Nature Genetics. While the researchers can’t say for sure why more men than women participated in the dispersion from Africa, the study’s lead author, Alon Keinan, notes that these findings are “in line with what anthropologists have taught us about hunter-gatherer populations, in which short distance migration is primarily by women and long distance migration primarily by men.” The study drew its logic from the fact that in a given population, the fraction of all chromosomes that are X chromosomes changes depending on the proportion of men. That in turn affects the rate at which mutations randomly spread through the X chromosomes relative to other chromosomes. One can detect the results of these processes by comparing the frequencies of specific gene variants in modern populations. The scientists say their method of comparing X chromosomes with the other non-gender specific chromosomes will be a powerful tool for future historical and anthropological studies, since it can illuminate differences in female and male populations that were inaccessible to previous methods. Keinan and colleagues noted that a few other processes could explain their findings, including that a tiny fraction of the women bore nearly all the children during the out-of-Africa dispersal, but that they seem unlikely. “Our observations could also be consistent with other sex-biased demographic processes, and future work should explore these scenarios,” they wrote.